When electronic products are becoming smaller in size and more intelligent with a high performance and a high reliability, requirements to integrated circuit (IC) package techniques are accordingly increased for higher integration of IC. Among these IC package techniques, wafer bumping is used to form solder bumps over an entire wafer on which integrated circuits have been built. After the wafer bumping process, the wafer is cut into individual chips for inner lead bonding. The wafer bumping is a critical step for device packaging, because the bumps formed on the wafer serve as electrical, mechanical, and mounting connections for flip-chip assemblies. These bumps must exhibit superior adhesion to the chips and minimum electrical resistance.
The bumps are formed by depositing solder alloy onto metal pads of the wafer. The deposited solder alloy is then performed a reflow process to reflow at a certain thermal profile including temperature above its melting point, form a metallic interconnect with the chips, and convert from its as-plated form into a spherical shape driven by liquid surface tension. Besides, the deposited solder alloy typically contains a native metal oxide layer on its surface. When the solder alloy is melted, the metal oxide layer remains in solid phase and acts as a skin. This constrains the molten solder surface and hinders the formation of uniform, spherical bumps. Therefore, it is also important to remove surface oxides from the deposited solder during the reflow process.
As aforementioned, since requirements to wafer bumping techniques are increased and higher, the thermal profile controlling and the metal oxide removal of the solder alloy during the reflow process have to be well controlled. Accordingly, improvements in reflow tools and methods of reflow continue to be sought.